Holographic interferometry has within a short time gained great importance for measuring the deformation of mechanic structures, for example in HNDT (Holographic Non-Destructive Testing).
The invention has the object of making it possible to produce interferometric information on the condition of an object, comprising exposure within short interval of light-recording material, for example photographic plates, films, thermoplastics, photoresistant or dichromatic materials and the like, hereinafter called hologram plates, and reconstruction of the exposed information on the hologram plates, by using a light source and a reference beam at the exposure in a known manner.
In the following, the principle of producing holograms is dealt with very briefly. It requires a light source with light of sufficient coherence, preferably a laser. It further requires a photographic material with high resolution which is sensitive to said light. A characteristic feature of the holographic technique is that no lens is required between the object and the photographic material. The light from a laser is caused to diverge and to light the object, from which the light in the form of more or less diffuse light (object beams) is thrown against the photographic emulsion, which usually is applied to a base consisting of glass or plastic film, the "hologram plate". At the same time, the hologram plate, receives additional light, the reference beam, preferably from the same laser. The reference beam preferably is produced by dividing light from the laser into several beams by some known optic method, for example a normal mirror, semi-transparent mirror. The hologram plate is exposed by causing the laser light to light for a certain time the object and the reference mirror, i.e. the mirror yielding the reference beam. The hologram plate thereafter is developed in a known manner, and when the plate thereafter again is lighted with laser light from about the same direction and with the same divergence as the reference light during the exposure, an image of the object is obtained, i.e. the hologram is reconstructed. This image is virtual, implying that it can be seen only through the hologram plate. When the direction of the light is entirely reversed, a real image is obtained, implying that the image can be caught on a screen. The virtual image has a fully three-dimensional effect and shows parallax, i.e. the object can be observed from different directions. When the object during exposure is reproduced on the hologram plate by means of a lens, a hologram image of the object is obtained at the reconstruction in the plane of the hologram plate. This image can be reconstructed also with usual incoherent light, for example from a light bulb. One condition for producing the hologram is that the spatial filter, object, reference mirror and hologram plate during the exposure do not move so much that the interference lines are erased which are formed when the object beam and reference beam meet on the hologram plate. A further condition is that the hologram plate has sufficient resolution for being able to reproduce these interference lines.
In double exposure interferometery first an exposure of the holographic plate is made. Thereafter the deformation to be examined is effected, for example by loading the object, whereafter a further exposure is made. It is important in this connection that the holographic set-up is so stable that neither the optic components, the hologram plate nor the object of examination have changed their positions between the two exposures. A movement of only some .mu.m is sufficient to destroy the measurements.
After the development and fixing of the hologram plate, when the plate is lighted by laser light (reconstruction), a three-dimensional image of the object measured is obtained, which image seems to be covered with interference lines, the light components of which connect the points, which have carried out between the exposures a movement of an integer number of half light-wave lengths (when the lighting, observation and movement have taken place along the same line).
In "real-time hologram interferometry," first one exposure is made, whereafter the hologram plate is developed and fixed and, finally, again is placed with high accuracy in the position which it occupied at the exposure. When no component of the set-up has been dislocated, no interference lines are seen when observing the object through the hologram plate. As soon, however, as the object is deformed, for example by being loaded, interference lines arise when observing the object through the hologram plate. This method, thus, has the advantage that the deformation of the object measured can be measured immediately, without requiring a further development of the hologram plate. This method, thus, makes it possible to study many different load cases within a short time. One disadvantage of the method is that the holographic set-up must remain in its position for the entire period required for carrying out the measurements and evaluations.
In "sandwich-holography," the different exposures are carried out on different hologram plates, which at the evaluation (reconstruction) are positioned adjacent one another in the form of a "sandwich". Prior to the first exposure two hologram plates are positioned in the same holder, one plate behind the other so, that the photographic emulsions do not lie adjacent each other. Prior to the second exposure these plates are replaced by two new ones, which are positioned in exactly the same manner. After all plates have been developed and fixed, they are placed together in pairs from the respective exposure, so that the plate having been foremost at the respective exposure also is foremost at the reconstruction. A thorough report on this technique is made in Applicant's articles "Sandwich Hologram Interferometry: a New Dimension in Holographic Comparison", Applied Optics, vol. 13, No. 9, September 1974, pages 2019-2025, and "Sandwich Hologram Interferometry. 2: Some Practical Calculations", Applied Optics, vol. 14, No. 4, April 1975, pages 981-984.
Advantages of the "sandwich-hologram" are that even after the experimental equipment has been dismantled, the following possibilites remain:
1. Different load combinations can be studied by the combination of different hologram plates. PA1 2. The effect of the total movement of the object can be eliminated, so that small local deformations can be studied in spite of large movement of the object. PA1 3. The interference lines can be manipulated so as to yield maximum information. PA1 4. Deformation signs (forward or rearward( can be obtained.